1. Technical Field
The present invention relates to a compact probe to test fiber optic lines that is capable of maintaining low transmission losses while the probe is moved relative to the line under test.
2. Background Art
In testing fiber optic lines, test cables are used to provide optical connection between the line under test and the test instrumentation. These test cables are comprised of a connector compatible with the line under test, an interface cable and a connector compatible with the test instrument. Since the fiber lines under test typically terminate in unique connectors, test technicians frequently must carry many different test cables in order to match the connectors to be tested in the field. The cost and inconvenience of maintaining a variety of test cables is a major disadvantage of current fiber optic test cables.
This compatibility problem is exacerbated by multipin connectors that are often used in fiber optic installations. As shown in FIG. 1, the fiber lines to be tested may terminate in a four pin male plug 100, which requires a test cable 105 that has a four pin female connector 110 that is compatible with the plug 100. Since there is a wide range of optical connector types and sizes, the costs of acquiring compatible test cables can be substantial.
In addition, current test cables are time-consuming to use and maintain. The process of properly mating male and female multipin connectors, such as a bayonet style connector, is very time-consuming. In addition, some types of connectors, such as a screw type connector, have mating surfaces that must be cleaned frequently to assure a secure connection. These are additional disadvantages of conventional fiber optic test cables.
As shown in FIG. 2, one solution that has been proposed is the use of a probe 200 consisting of a single fiber optic cable 205 terminated in a probe ferrule 210 that is mounted in a grip 215. When the probe 200 is inserted in a connector 220 to be tested, it typically engages a sleeve 225, made of for example ceramic or metal, in which a test ferrule 230 is mounted. An exemplary probe of this design is made by Glenair Corporation of Glendale, Calif.
The use of a probe of this design displays several disadvantages. When a user inserts the probe 200 into the connector 220, a rotation motion about the axis of the probe 200 frequently occurs which causes the faces of the probe ferrule 210 and test ferrule 230 to grind against one another Such a grinding motion easily scratches the optical transmission surfaces of the ferrules 210 and 230, which increases transmission losses.
This grinding can also affect reliability of the probe 200 because of the stresses that can arise at the contacting surfaces of the ferrules 210 and 230. These surfaces consist primarily of the glass cores of the fiber optic cable, which may have a diameter as small as 8-9 microns. Consequently, even modest shear forces from rotation will produce high localized stress over such small contacting area. These stresses can cause cracking of the glass core and protective cladding and failure of the optical connection.
In addition, transverse (or skew) forces F may be applied to the probe 200 during the insertion or test process. The affect of these forces F on the ferrules 210 and 230 is increased because the forces are applied to the grip 215 of the probe 200, which results in a mechanical advantage compared to the length of the ferrules. These forces F will cause axial misalignment of the ferrules 210 and 230 that degrades the optical connection between the ferrules and results in transmission losses. This result jeopardizes the integrity of test process. Further, if the forces F are large, the sleeve 225 may break rendering the connector 220 useless.
An additional disadvantage of probes of this design is that longitudinal forces on the grip 215 are transmitted to the ferrule 210. Since the ferrule 230 is, stationary compression forces will develop and the contacting surfaces of the ferrules 210 and 230, which forces can fracture the glass cores of the fiber optic cables.